The present invention relates generally to a zoom optical system used with image pickup devices, and more particularly to a zoom optical system for use with photographic cameras, especially lens shutter cameras.
Cameras having zoom lenses are now popular for recent lens shutter cameras, and there are growing demands for cameras equipped with a compact phototaking lens having an increased zoom ratio.
Generally, a zoom optical system having a zoom ratio of about 2 to 3 is constructed in the form of a two-group one consisting of, in order from its object side, a lens group having positive refracting power and a lens group having negative refracting power, and a zoom optical system having a zoom ratio of about 3 to 4.5 is set up in the form of a three-group one consisting of, in order from its object side, a lens group having positive refracting power, a lens group having positive refracting power and a lens group having negative refracting power.
In general, two-group zoom arrangements are used especially for lens shutter cameras targeted at low cost and compactness, because of comprising a simple mechanical mechanism with reduced sensitivities of the adjacent lens groups to decentration.
Some two-group zoom lens systems having a zoom ratio of about 3 are generally designed with an angle of view of about 60° at the wide-angle end. On account of growing needs for scenic shots taken at wider-angle ends, which are a reflection of recent going-abroad booms, and for lens shutter cameras with wide-angle zoom lenses at oversea markets, however, there is an increasing demand for two-group zoom lenses having an angle of view of about 70° or greater at the wide-angle ends and having a zoom ratio of about 3.
With zoom lenses having a zoom ratio of about 3 while maintaining an angle of view of about 70° or greater at the wide-angle ends, however, it is difficult to maintain image-formation capability at the telephoto ends.
Lens arrangements so far proposed to overcome such problems are disclosed in JP-A's 11-14903, 2000-28921 and 2000-137163.
However, the lens arrangements set forth in JP-A's 11-14903 and 2000-137163 have a zoom ratio of as low as 2.8 although the angle of view at the wide-angle ends is of the order of 70° or greater. The lens arrangement of JP-A 2000-28921, too, has an zoom ratio of at most 3 although the angle of view at the wide-angle end is 70° or greater.